Method and apparatus for continuous casting utilizing solidified skin thickness determinations



Sept. 6, 1966 A. THALMANN 3, 70

METHOD AND APPARATUS FOR CONTINUOUS CASTING UTILIZING SOLIDIFIED SKIN THICKNESS DETERMINATIONS Filed Oct. 29, 1962 15 Sheets-Sheet 1 Fig. 1

S 20 I c Sept. 6, 1966 A. THALMANN 3,270,376

METHOD AND APPARATUS FOR CONTINUOUS CASTING UTILIZING SOLIDIFIED SKIN THICKNESS DETERMINATIONS Filed Oct. 29. 1962 15 Sheets-Sheet 2 x32 F'A7 M *4 36 A12 A13 1 1 Ag A8 Sept. 6, 1966 A. THALMANN 3,270,3

METHQD AND APPARATUS FOR CONTINUOUS CASTING UTILIZING SOLIDIFIED SKIN THICKNESS DETERMINATIONS Filed 001;. 29, 1962 15 Sheets-Sheet 3 Se t. 6, 1966 A. THALMANN 3,270,375

METHOD AND APPARATUS FOR CONTINUOUS CASTING UTILIZING SOLIDIFIED SKIN THICKNESS DETERMINATIONS Filed Oct. 29, 1962 13 Sheets-Sheet 4 Sept. 6, 1966 A. THALMANN 3,270,376

METHOD AND APPARATUS FOR CONTINUOUS CASTING UTILIZING SOLIDIFIED SKIN THICKNESS DETERMINATIONS Filed Oct. 29, 1962 15 Sheets-Sheet 5 B2 k k Sept. 6, 1966 A. THALMANN 3,270,376

METHOD AND APPARATUS FOR CONTINUOUS CASTING UTILIZING SOLIDIFIED SKIN THICKNESS DETERMINATIONS Filed Oct. 29, 1962 l5 Sheets-Sheet 6 -c7 c5 c4 Sept. 6, 1966 Filed 001;. 29. 1962 A. THALMANN METHOD AND APPARATUS FOR CONTINUOUS CASTING UTILIZING SOLIDIFIED SKIN THICKNESS DETERMINATIONS l3 Sheets-Sheet 7 Fig.4c C1 I I IIII IIII I IIII II I I IIII IIIIIIIIII I IIIIIIIIII IIIII (:2 H Nil Nil ||1Hll1l1l1li| C5 [1 1-1 1 1 nm IL JL J I J M (:7 n H H HF P pw 1f Fig.6

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ME AND APPARATUS FOR CONTINU CASTING UTILIZING SOLIDIFIED SKIN THICKNESS ERMINATIONS Filed Oct. 29. 1962 13 Sheets-Sheet 8 Fig. 5a --97 D6 9:3 v 231. g5 96 J I m- 02 '03 D4 I D7 g x 1,08 409 Fig.5b

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METHOD AND APPARATUS FOR CONTINUOUS CASTING UTILIZING SOLIDIFIED SKIN THICKNESS DETERMINATIONS Filed Oct. 29, 1962 15 Sheets-Sheet 10 Fig. 9

Sept. 6, 1966 A. THALMANN 3,

METHOD AND APPARATUS FOR CONTINUOUS CASTING UTILIZING SOLIDIFIED SKIN THICKNESS DETERMINATIONS Filed 001;. 29, 1962 15 ShGGtS-SIIGGt 11 Dept. 6, 1966 A. THALMANN 3,270,376

METHOD AND APPARATUS FOR CONTINUOUS CASTING UTILIZING SOLIDIFIED SKIN THICKNESS DETERMINATIONS Filed Oct. 29, 1962 13 Sheets-Sheet 12 Se t. 6, 1966 A. THALMANN 3,270,376

METHOD AND APPARATUS FOR CONTINUOUS CASTING UTILIZING SOLIDIFIED SKIN THICKNESS DETERMINATIONS Filed 001;. 29, 1962 15 Sheets-Sheet 13 Fig. 10a

United States Patent 3,270,376 METHOD AND APPARATUS FOR CONTINUOUS CASTING UTILIZING SOLIDIFIED SKIN THICK- NESS DETEINATIONS Armin Thalmann, Uster, Zurich, Switzerland, assignor to Concast A.G., Zurich, Switzerland Filed Oct. 29, 1962,.Ser. No. 233,723 Claims priority, application Switzerland, Nov. 4, 1961, 12,794/ 61 Claims. (Cl. 22-57.2)

The output of a continuous casting machine depends from the material and the section to be cast, from the I quality thereof, the heat absorption capacity of the plant etc. By means of known empirical figures, it is possible to determinate the values for the required cooling and for the optimum casting speed. The liquid metal core or pool thereby represents a measure for these values.

This core is surrounded by a solidified external zone called the skin or crust of the strand and if the shape of the core is considered as a measure for the cooling and the casting speed then also the thickness of the skin will be a measure for these values. If the casting speed and the cooling are constant, then theoretically the thickness of the skin at a determinated level in the range of the core also should always be the same. But reduction of the casting temperature during the casting operation, the adjustment of the lowering speed due to modification-s in the feed of the liquid casting material and other factors may, however, modify the shape of the pool and thus also the thickness of the solidified skin. If the skin thickness falls short of a certain value, more particularly in the zone immediately following the die or mould the danger appears of the feared breakouts which may put out of operation the machine for a longer period. Moreover, especially when larger sections are cast swellings and bellyings may appear under the action of the ferrostatica-l pressure due to an all too thin skin below the roller guiding. Such bellyings make the cast material improper for further use.

It has been tried to avoid these drawbacks by reducing the lowering speed to such an extent, that by the safety margin thereby obtained for the skin thickness the appearance of the mentioned difficulties is reduced. However, it is obvious that .any reduction of the lowering speed involves a reduction of the output of the whole machine.

It is a prime object of the present invention to provide a method and a device avoiding the mentioned danger of break-outs without such undesirable reduction of the output of the machine.

To this end, the invention contemplates a method for continuous casting comprising the steps of ascertaining during the casting operation, and in a range in which the strand shall have a predeterminated skin thickness in accordance with the requirements of the material, at least periodically this skin thickness and of using the measured value-s for controlling the machine in the sense of these requirements.

Advantageously this is obtained in the manner that the thus measured actual skin thickness is compared with a nominal value determinated by the desired skin thickness, whereby the machine is controlled by means of the appearing deviations of the actual value from the nominal value.

Thus this method makes possible for an optimum use of the machine in that it does not only measure the skin thickness but moreover compares the obtained values with a nominal value for the skin thickness and makes use of the produced signals for controlling the machine. The signals for this control are continuously produced during the casting operation.

The obtained values for the deviations of the actual skin thickness from the nominal value therefor may be used to control the lowering speed, the cooling intensity or both these factors. Of course the same signals may be used to control other factors, such as for example the feed of liquid metal.

iln the selected range, for example at the outlet of the die, waves, electric current or rays are sent transversally through the strand and give indications on the transitions from the .solid to the liquid state within the portions of the strand penetrated thereby, these indications being given in the form of signals.

It is well known that the electrical conductivity of material, e.g. of steel, is dilierent whether this material is in solid or liquid state. If, now, in the mentioned range, an electrical current is sent perpendicularly to the axis of the strand through the latter the differences of the conductivity may be used to localize the transitions from solid to liquid and may be compared with a nominal value, the signal obtained from this comparison being amplified before it is transmitted as a control value to the machine.

Radioactive rays, for example gamma-rays have the property to be responsive to the density of a material. If, still in the mentioned range, an emitter, for example a source of cobalt-60, is arranged on one side of the strand while opposite thereof there is arranged a receiver, a difference in the received intensity will appear on the receiving side due to the different densities of the liquid and of the solid material. This difference may be used to localize the transition from solid to liquid in the material of the strand and by comparing the nominal value of the skin thickness with this localized point, for example in a discriminator, a signal may be produced that may be used for controlling the machine after amplification.

For localizing the transitions from solid to liquid of a material, it is also possible to use waves, whereby waves of high frequency such as for example waves produced by ultrasonic emitters, are particularly appropriate. An ultrasonic emitter placed in the selected range on one side of the casting emits high frequency waves transversally to the casting axis. These waves are reflected at the points of transition from solid to liquid. This reflection generally designated by echo is received in a receiver placed on the same side of the strand. The time intervals between the emission of the pulses and their reception, constitute an indication for the point of reflection. Such time intervals may be compared in a discriminator with a nominal value, whereby the produced signals after amplification serve to controlthe machine.

Moreover, it is possible to localize the points of transiti-on from solid to liquid by letting the Waves pass through the strand (method of penetration). According to this method an emitter is placed in the selected range on one side of the strand and a receiver is placed in the same range but opposite to the emitter with respect to the strand. By measuring the time required by the ultrasonic waves for passing through the strand it is possible to localize the points of transition from solid to liquid in the strand. Again then this point is compared in a discriminator with a nominal value for the skin thickness and the signals produced by any deviation are fed to the machine for control purposes after amplification.

Practice has shown, however, that ultrasonic devices are sensitive to high temperatures, so that a direct coupling of such devices to the hot strand cannot be envisaged. Therefore it is proposed to place the ultrasonic emitter and the ultrasonic receiver within water-cooled rollers pressed against the strand whereby the cooling water may serve as coupling liquid between the Wave emitter and the rollers. The transmission of the waves from the rollers to the strand is effected by direct contact between these two parts. However, the scale appearing on the surface of the strand may to a certain extent hinder such transmission of the ultrasonic waves into the strand. This may be prevented by guiding a part of the cooling water serving to the direct cooling of the strand between the strand and the roller. This cooling water then serves as a coupling liquid between the strand and the rollers.

A further possibility of coupling the ultrasonic waves may consist in the use of a jet of water under pressure between the emitter and the receiver on the one hand and the hot strand on the other hand. This jet is directed to impact perpendicularly onto the strand with a pressure that makes the water pierce through the vaporous layer on the casting surfaces to be described hereinafter, due to the kinetic energy imparted to the jet by this pressure.

It is well known that a layer of vapor is produced between a surface of high temperature and a liquid. This is generally known under the designation of phenomenon of Leidenfrost. This vapor layer on the casting surface may render impossible the transmission of the ultrasonic Waves from the coupling liquid to the casting. It is possible to reduce this insulating vapor layer by applying a voltage between the liquid and the hot surface.

As a material solidifies there is generally no sharp border between the liquid and the solid state. This is the reason for the formation of dendrites growing in direction of the casting axis. Moreover, it is well known that a stick-ing between the strand and the inner die wall cannot be avoided completely in spite of die oscillation and lubrification. Now this sticking stresses the skin of a strand at every return stroke of the die, this stress being a tensioning load. In view of the weak solidity and the large dilatability of the thin skin that just has solidified, this skin is submitted to a dilatation which in turn leads to irregularities in the skin thickness. These irregular borders at the solidification front also lead to irregularities of the values localizing the transition from solid to liquid which in turn has a disadvantageous eflFect due to the instability of the control .produced thereby. This may even lead to permanent modifications of the adjusting functions. Such instability of the control may be prevented by controlling the machine by the measured deviations in pre-adjustable intervals only. There are several ways for producing or obtaining such intervals. For example the actual skin thickness may be measured at regular time intervals only. Also it is possible to release by the deviation of the measured actual skin thickness from its nominal value a time element which releases a signal for controlling the machine after its proper delay time only. Further it is possible to feed the deviation of the measured actual skin thickness from its nominal value to a counting device which releases a signal to the control only after a pre-selected value has been attained.

In order to obtain a precise picture of the pool or liquid core in the strand it may be advantageous to measure the skin thickness at several points in the range of this core. The obtained values make possible by interpolation to figure out the shape and size of the core. It is also possible to compare the obtained measurements for the skin thickness with each other. For example the sum of all measured skin thicknesses may be compared with the sum of the nominal values, or the signal for the smallest skin thickness may be found out and only after such a result is present, the machine may be controlled or the control of the machine may be influenced.

With the received deviation signals of the skin thickness it is possible to influence the adjusting functions. However, the requirements of the material may ask for further parameters influencing the adjustment to be considered. Such parameters are the temperature of the casting surface, the temperature of the liquid metal previous to its arrival into the die, etc. The measured actual values of these parameters may again be compared with appropriate nominal values in order to influence, by means of appearing deviations the control of the machine by the deviations of the actual from the nominal skin thickness.

Other features and advantages of the invention will become apparent from the description, now to follow, of preferred embodiments thereof, given by way of example only, and in which reference will be made to the accompanying drawings, in which:

FIGURE 1 diagrammatically illustrates a continuous casting machine;

FIGURE 2 shows the arrangement of an emitter and of a receiver for carrying out the ultrasonic echo method;

FIGURE 20 shows a sequence of pulses;

FIGURE 2b shows a block diagram illustrating the production of control pulses in the echo method;

FIGURE 20 shows a series of signal forms;

FIGURE 3 shows the arrangement of emitter and receiver for carrying out the ultrasonic method of penetration with a coupling by water jets and ionization of the vapor layer;

FIGURE 3a shows a sequence of pulses;

FIGURE 3b is a block diagram illustrating the production of control pulses according to this method;

FIGURE 3c shows forms of signals obtained thereby;

FIGURE 4 shows the arrangement of emitter and receiver in the method using gamma-rays passing through the strand;

FIGURE 44: shows a sequence of signals;

FIGURE 41) is a block diagram illustrating the production of control pulses according to this method;

FIGURE 40 shows forms of signals;

FIGURE 5 illustrates a method using electrical resistance;

FIGURE 5a is a block diagram illustrating the production of control pulses according to this method;

FIGURE 5b shows signal shapes;

FIGURE 6 illustrates the determination of the surface temperature of the strand by means of a thermo-element;

FIGURE 6a is a block diagram illustrating the production of temperature signals;

FIGURE 7 shows the determination of the surface temperature of the strand by means of a ray-pyrometer;

FIGURE 7a is a block diagram showing the production of temperature signals in accordance with this last method;

FIGURE 8 is a block diagram illustrating the evaluation of several measurements of the skin thickness;

FIGURE 9 is a block diagram showing the evaluation of the pulses according to FIGURES 2 and 3;

FIGURE shows signal forms;

FIGURE 10 is a block diagram illustrating the evaluation of the signals according to FIGURES 4 and 5, and

FIGURE 10a shows signal forms.

FIGURE 1 This figure represents a conventional continuous casting machine to which the invention may be applied. Reference numeral 11 designates a lad-1e containing liquid metal for example liquid steel that is poured into an intermediate container or tundish 12. From the tundish 12 the liquid metal is guided into an oscillating, Watercooled die 13 in which is formed a solidified external zone the so-called crust or skin. The liquid metal enclosed within this skin is generally referred to as pool or liquid core. The skin and the core together form a strand 14 which is drawn from the die 13 in well known manner at a determinated speed first by means of a dummy-bar I9. Thereby the strand is cooled in a secondary cooling zone 15 through which it is guided. By the cooling to which the strand is submitted in this secondary cooling zone, it solidifies more and more, i.e. the liquid core becomes smaller and smaller. The solidified strand 14 is drawn off by a set of rollers 16 and is deviated by a bending mechanism 17. A straightening device 18 finally straightens the strand again whereafter it is cut into lengths.

Detection means 20 for determinating the skin thickness are advantageous-1y arranged immediately after the die or mould 13. They may of course also be placed at any other point or at several points in the range of the liquid core.

FIGURES 2-2c In FIGURE 2 the detection means 20 is shown in form of an ultrasonic emitter 21 and receiver 22 lodged.

in a water-cooled roller 23. This roller 23 is pressed by means of a pressure device (not represented) against the strand 14 including in its interior the liquid core'S. Between the strand 14 and the roller 23 the cooling water 24 of the secondary cooling is dammed in the roller 23 and the cooling water 24 forms the coupling fluid for the ultrasonic waves between the emitter 21, the strand 14 and the receiver 22.

The emitter 21 emits ultrasonic Waves in direction towards the strand 14. The waves are reflected towards the receiver 22 at the points of transition a-e between different fluid states so that there is obtained a sequence of pulses as shown in FIG. 2a. The emission of such waves is repeated at regular time intervals t1 with a pulse 0. The reflection of this pulse at point a (transition from the cooling water for the roller to the roller) supplies in the receiver an echo pulse 1, at the point b (transition between the roller and the strand surface) an echo pulse 2, at the point (transition from solid to liquid steel) an echo pulse 3, at the point d (transition from liquid to solid) an echo pulse 4 and at the point e (strand surface) an echo pulse 5. The impact of the pulses on the receiver 22 takes place at time intervals t. The time interval between the emission of pulse 0 and the receipt of the echo pulses is -a measure for the distances covered by the waves. For example a time interval- 14 between the pulses 2, 3 and 4, 5, respectively, corresponds to a skin thickness K between the points b, c and d, e, respectively. In order to simplify the present example for the following description, only the pulses 2 and 3 will be considered hereafter. In order to determinate the time interval t4 it is necessary to introduce internal control time intervals 12 and t3. The time interval 12 corresponds to the interval between the emission of pulse 0 and an assumed time point P between the echo pulses 1 and 2, while the time interval 13 extends from said point P to an assumed time point Q between the pulses 3 and 4.

The pulse sequence according to FIGURE 2a is now evaluated to form control signals corresponding to the skin thickness, by means of a diagram illustrated in FIG- URE 2b.

The signal forms appearing at the outlets of the blocks are shown in FIGURE 20, whereby it is assumed for the signal sequence at the left a measured actual skin thickness inferior to the nominal skin thickness and for the signal sequence at the right a measured actual skin thickness superior to the nominal skin thickness.

A pulse generator 30 produces the tension pulse 0 which, as already mentioned, is repeated at regular time intervals t1. These tension pulses 0 on the one hand act onto an ultrasonic emitter 21 for example onto an emitter with an oscillating quartz and on the other hand onto time elements 31 and 32. In the emitter 21 these pulses are transformed into ultrasonic waves. The time elements 31 and 32 are switched-in by the pulses A1 of the generator 30 during the time intervals 12 and (t2+t3), respectively and they issue at their outlets signals A4 and A5, respectively. The signals A4 and A of the time elements 31 and 32, respectively, act onto a 0-1-implication member 33 producing at its outlet a signal A6 only when at its inlet the signal A4 is zero and simultaneously the signal A5 is one. The signal A6 thus extends over the time interval t3.

The echo pulses 1-5 are received by the receiver 22, which may be a dynamical microphone. The outlet signals A2 of the receiver 22 are amplified by an amplifier 34 to a level A3. The signals A3 and the signal A6 act onto a gate 35. The signal A3 may appear at the outlet of the gate 35 as signal A7 only when the signal A6 with the value 1 is present. Consequently pulses may pass through the gate 35 only during the time interval t3, ie the echo pulses 2 and 3 which are contained in the outlet signal A7 may pass through. This signal A7 now acts onto both inlets of a bistable sweep stage 36 which is switched-in by the echo pulse 2 and switched-off by the echo pulse 3. Thus the outlet signal A8 with the switch-in time t4 and [4, respectively, represents the actual value of the skin thickness K.

To use this actual value for controlling purposes it is necessary first to introduce a nominal value for the skin thickness. To this end the outlet signal A8 acts onto a differentiating member 37, the outlet signal A9 of which leads to a rectifier 38 which cuts off the negative portion of this signal A9 leading to the production of a signal A10. With this signal A10 there is switched in a time element 39 for the nominal value with adjustable delay time t5. The outlet signal All of this time element 39 with the delay time t5 thus represents the nominal value of the skin thickness K. r In the further processing of the control the actual value and the nominal value are discriminated. Thereby the signal A8 for the actual value and the signal A11 for the nominal value act onto implication members 40 and 41. The 1-0-implication member 40 supplies at its outlet a signal A12 only provided that the signal A8 for the actual value is present and simultaneously the signal A11 for the nominal value disappears, in other words each time I4 is superior to t5. Analogously the O-I-implication member 41 only supplies at its outlet a signal A13 when the signal A8 for the actual value disappears and simultaneously the signal A11 for the nominal value is present, i.e. when t4 is inferior to t5. Consequently the outlet A12 only supplies a signal when the skin thickness K exceeds the nominal value. The outlet A13 further supplies a signal only when the actual skin thickness is inferior to the nominal value therefor. The pulse duration of these signals is directly proportional to the deviation from the nominal value. Whenever no signal appears at the outlets A12 and A13 the skin thickness corresponds to the nominal value.

In the embodiment which just has been described, the actual skin thickness is measured continuously. In order to avoid an instability of the control that may result therefrom, it is suggested to determine these signals at regular time intervals. This may be obtained by switching a time element (not represented) into the circuit before to the pulse generator 30, which switches in the generator for a short tire only and leaves it switched off hereafter for a longer period.

FIGURES 3-30 FIGURE 3 illustrates the method of penetration. An emitter 50 and a receiver 51 are each lodged in a water container 52 and 53, respectively. Each of these containers 52 and 53 is provided with a jet nozzle by means of which it is possible to direct a jet of cooling water 54 and 55, respectively under such pressure against the strand 14 that this jet may be used as coupling means for transmitting ultrasonic waves to the strand.

In order to reduce the influence of the phenomenon of Leidenfrost, sources of current 56 and 57 are connected on the one hand, with water housings 52 and 53, respectively, which are electrically insulated from the strand, and on the other hand with the contact rollers 58 and 59, respectively. Between the cooling water jets 54 and 55, respectively, and the hot surface of the strand a vapor film or layer is formed. This hot surface and the jets of water 54 and 55, respectively, serve as electrodes separated from each other :by the vapor film acting as insu lator. By applying a voltage between the electrodes a high electrical field strength is produced in the very thin vapor film leading to gas discharges. Thereby the vapor molecules are ionized and wander back in the liquid. By adjusting the applied voltage it is possible to reduce the vapor film to zero so that the transmission of ultrasonic Waves is not hindered anymore.

The emitter 50 emits ultrasonic waves in direction to the strand 14 and to the receiver 51. There is formed a pulse sequence according to FIGURE 3a. The emission of these waves takes place at regular time intervals :10 by means of a pulse The impact of a pulse 1' representing the emission pulse 0 delayed by its passage through the material of the strand on the receiver 51 takes place after a time interval t. This interval between the emission of the pulse 0' and the receipt of pulse 1' thus constitutes a measure for the dimension of the liquid core S plus both skin thicknesses K.

In order to ascertain the two skin thicknesses, it is necessary to introduce further time intervals r20, I30 and M0. The time interval 120 represents the interval from the emission pulse 0' to an assumed time point R immediately before pulse 1', while the time interval 230 extends from this time point R to an assumed time point M immediately after the pulse 1. The time interval t40 begins at point R and ends at pulse 1.

The pulse sequence according to FIGURE 3a is now transformed according to the block diagram of FIGURE 3b into control signals corresponding to the skin thickness. The associated signal forms are shown in FIGURE 30, whereby for the signals represented at the left there is assumed an actual skin thickness too small with regard to the nominal skin thickness While at the right in this figure, there is assumed that the actual skin thickness is superior, to the nominal thickness.

The evaluation of the pulse sequence according to FIGURE 3a is effected in the same manner as has already been described for FIGURE 2b, whereby a ferromagnetical vibrator is used in the emitter 50 in lieu of the oscillating quartz in the emitter 21 and a crystal microphone is used in the receiver 51 in place of the dynamical microphone in the receiver 22. The only difference with respect to FIGURE 2b remains solely in the switch-in triggering of the bistable sweep stage 36. While this is effected in FIGURE 2b by signal A7 of the prior stage 35, in FIGURE 3b a switch-in signal B must be separately produced since there is no corresponding pulse. To this end the output signal A6 of the O-l-implication member 33 is transformed into a signal B14 by means of a differentiating member 59. By a rectifier 60 the negative portion of the signal B14 is cut off so that the mentioned switch-in signal B in form of a signal B15 is produced. The further production of the signals B8 to B13 is effected similarly to that of the signals A8 to A13 in FIGURE 2b.

FIGURES 4-40 In FIGURE 4 the detection means 20 consists of a source of rays 70, for example of a source of 00-60 and of a radioactivity responsive element 71, for example a Geiger counter. In order to protect this counter 71 from heat, it is surrounded by a cooling mantle 69. The gamma-rays emitted by the source 70 penetrate through the strand 14 and are transformed in the Geiger counter 71 into a continuous pulse sequence having as parameter the time I according to FIGURE 40. The rate of admission of the gamma-rays in the Geiger counter 71 varies With modifications of the density in the penetrated strand 14 and more particularly with any modification of the thickness K of the skins and consequently with the size of the liquid core S.

The evaluation of this pulse sequence is effected according to the block diagram of FIGURE 4b. The signal forms appearing at the Outlets of the blocks are shown in FIGURE 40, whereby on the left side thereof the actual skin thickness is larger than the nominal value and on the right side thereof, the actual skin thickness is smaller than the nominal value. The pulses C1 supplied by the Geiger counter 7 are amplified to a level C2 by means of an amplifier 72. These outlet pulses O2 act onto a pulse transformer '73 which lets pass only a predetermined part of the pulses, for example only each third pulse. The average number of outlet pulses C3 in a time interval t constitutes a measure for the actual value of the skin thickness K, i.e. the outlet signal C3 by its frequency of pulses represents the actual value of the skin thickness.

In order to make use of this actual value for control purposes this actual value must first be compared with a nominal value for the skin thickness. To this end there is provided a generator for the nominal value pulses 74 and the outlet signal C4 of this generator 74 acts onto a NOT gate 75. The outlet signal C5 of this gate, by its frequency of pulses, represents the desired nominal value for the skin thickness.

The actual and the nominal value are discriminated in the further processing of the control. Thereby the actual valve signal C3 and the nominal value signal C5 act onto an integrating member 76 the outlet signal C6 of which is formed as follows:

The signals 03 and 05 respectively, owing to their opposite polarity, lead to an addition or subtraction, respectively of the voltage of a storing element contained in the integrating member 76. The thus produced outlet signal C6, which according to the conditions may have a positive or a negative value, is brought by an amplifier 77 to the level C7. The outlet signal C7 of the amplifier acts onto Schmidt triggers 7 8 and 79. The trigger 78 forms an outlet signal 08 only when signal C7 is larger than a predetermined positive value p, i.e. when the skin thickness K is too small. On the other hand an outlet signal C9 can only be formed at the trigger 79 when the signal C7 exceeds a predetermined negative value n, i.e. when the skin thickness K is too large. If no signals C8 or C9 appear, this means that the skin thickness K actually corresponds to its nominal value.

FIGURES 5-56 FIGURE 5 illustrates a method based on the electrical conductivity, in which contact rollers 99 and 91 are pressed against the strand 14. These contact rollers are supplied with current from an evaluation circuit 92 and this current passes through the strand 14. In accordance with the thickness of the two skins K and consequently with the size of the liquid core S the electrical resistance between the contact rollers and 91 varies.

The evaluation circuit is shown in form of a block diagram in FIGURE 5a. All signal forms appearing at the outlet of the blocks are shown in FIGURE 5 b.

A resistance measuring bridge 94 in form of a Wheatstone or Thomson bridge is supplied from a rectangular generator 93 the output signal of which is designated by D1. In a branch of the bridge the resistance produced by the strand 14 between the contact rollers 90 and '91 is contained as a measure for the actual value of the skin thickness K. In the other branch of the bridge there is arranged a potentiometer serving to assign a measure for the nominal value of the skin. Simultaneously this bridge serves as discriminator since the outlet signal D2 with its phase position and its amplitude indicates whether the skin thickness is too large or too small as compared with the nominal value for the skin thickness. Whenever no signal D2 appears, i.e. when the bridge 94 is in equilibrium the skin thickness K is equal to the nominal value.

In order to transform the AC. bridge signal D2 into a continuous signal the alternate voltage D2 is fed to an amplifier 95 the out-let signal D3 of which on the one hand acts onto a limiting stage 96 adapted to limit the amplitude of signal D3 to a predetermined value b and on .the other hand onto a Gratz rectifier 97. The outlet signal D4 of the limiting stage 96 leads to a gate stage 98 supplying an outlet signal D5 only when the outlet signal D6 of the rectifier 97 attains a predetermined value s. This last-mentioned value must be selected in 9 such manner, that the gate stage 98 is open only when the inlet signal D 4 is already limited by the element 96. The outlet signal D5 acts onto phase discriminators 99 and 100. [Further .the phase discriminator 100 is also acted upon by the outlet signal D1 of the rectangular generator 93. Further the phase discriminator 99 receives the outlet signal D7 of a NOT-gate 101, to the inlet of which is fed the signal D1. The discriminators '99 and 100, respectively, have the property to produce an outlet signal D8 and D9, respectively, only when both their inlet signals have equal phases. If the phases are not equal or if a phase falls out, no outlet signal is produced. The appearance of an outlet signal D8 and D9, respectively, indicates a deviation of the skin thickness K from the nominal value thereof. If no signal appears, the actual skin thickness corresponds to the nominal value therefor.

In FIGURES 25 the actual skin thickness is always compared with a nominal value for this thickness. The signals produced by differences between these two values shall subsequently be transformed into control signals for adjusting the machine. This transformation may be effected by hand or automatically.

For the manual control two solutions are possible. Either the signals indicating some difference between the actual value of the skin thickness and the nominal value therefor may be directly brought in form of light signals to the control board of the operator. Thereby, in order to prevent a fliminering of the light signals due to frequent changes in the signals it may be of great advantage to provide, as already mentioned, a timed locking of the action of the signals.

Or the signals indicating the actual skin thickness may be fed to an oscillograph mounted in the control table of the die operator without being compared with a nominal value, whereby the actual skin thickness appears on the screen of the oscillograph in form of a curve. A marking brought on the screen indicates the nominal skin thickness and should the curve of the actual skin thickness depart from this marking the die operator may directly make the required changes in the control.

Instead of being supplied to an oscillograph the signals representing the actual skin thickness may be fed to an indicating instrument. In order to eliminate the influences of fluctuations of the actual value, a marking on this instrument indicates a nominal range with a minimum and a maximum nominal value. Correction of the machine by the die operator takes place only when the hand of the instrument leaves this nominal range.

For the automatic transformation of the deviation signals the latter are fed to an evaluating circuit releasing the adjusting functions. For the next following em- 'bodiments of such an evaluating circuit an additional condition shall be introduced.

It is known, that some steel qualities are very sensitive to cracking and this cracking generally is closely related to the intensity of cool-ing. The temperature at the strand surface constitutes a measure for this cooling intensity and it may, for this reason and for some steel qualities be advantageous to take into consideration in the functions of adjustment, the temperature of the strand surface, this being the additional condition just mentioned before.

FIGURES 6 and 6a In order to ascertain the surface temperature T of the strand 14 in FIGURE 6 there is provided a platinumrhodium thermoelement 110 scanning with its first soldering point 111 the surface of the strand 14 and icontacting with its second soldering point 112 a heat element 113. By a current source 114 and an intermediate nominal value potentiometer 115 the heating element 113 is brought to a desired nominal temperature. In accordance with the point of greater temperature at the one or at the other soldering points there is produced a positive or a negative voltage applied to an evaluating circuit 116. In the case of equality between nominal and equal temperature this voltage disappears.

FIGURE 6a illustrates this evaluating circuit 116 in a block diagram. The very small thermo-electrical voltages E1 are amplified to a level E2 by means of an amplifier 117, this outlet signal E2 actuating Schmidt triggers 118 and 119. The operation of a Schmidt trigger has already been described with respect to FIGURE 4. The appearance of outlet signals E3 and E4 at the triggers 118 and 119, respectively, indicates deviations of the actual temperature from the nominal temperature. No signal appears each time the actual temperature coincides with the nominal temperature.

It is also possible to determine the surface temperature T at several points of the strand whereby it exerts separately or by means of an average value its influence onto the control.

FIGURES 7 and 7a In order to ascertain the surface temperature T of the strand 14 use may also be made of a ray-pyrometer such as shown for example in FIGURE 6. To this end the temperature radiation of the strand surface is directed through a lens onto a first photo-electric resistance mounted into an evaluation circuit 131. The temperature radiation of a further source of heat 132 is directed through a lens 133 onto a second photo-electric resist ance. This last-mentioned source of heat 132 is maintained at the desired nominal temperature by a source of current 134 by means of a nominal value potentiometer 135.

In FIGURE 70 the evaluating circuit 131 is shown by way of a block diagram. The latter corresponds to that of FIGURE 5a with the exception that instead of the measuring bridge 94 having resistances in the branches of the bridge, there is provided in this example a measuring bridge 136 having mounted into its branches the two mentioned photo-electric resistances. The appearance of outlet signals F8 and F9 corresponding to the outlet signal-s D8 and D9, respectively, in FIGURE 5, indicates a deviation of the actual temperature from the nominal temperature. No signal appears when both these temperatures are equal.

FIGURE 8 This figure illustrates the comparison of three measurements of the skin thickness taken at diflerent places in the range of the liquid core for the purpose of producing a single signal for controlling the machine. Thereby two possibilities of comparison shall be described:

(a) a control signal shall appear only when all three measurements depart in positive or in negative direction,

(b) a control signal shall only be produced when only one of the three measurements is positive or negative.

The possibilities of comparison may be selected by a program switch 154. The illustrated position of switch 144 corresponds to the second of the above-mentioned possibilities (case b). The signals G1, G2, G3 for too thick skins act onto an OR-gate 140 and onto an AND- gate 142, while the signals G4, G5, G6 for too thin skin-s are directed to an OR-gate 141 and to an AND-gate 143. An outlet signal G7 may only be produced at gate 140 when one of the signals G1, G2, G3 is present and analogously an outlet signal G8 may only be produced at gate 141 when any of the signals G4, G5, G6 is present. A signal G9 at gate 142 can only be produced when all three signals G1, G2, G3 are present, while a signal G10 at gate 143 canonly be produced when all of the signals G4, G5, G6 are present. Consequently the signals G11 and G12, respectively, form the mentioned single signal for the control.

FIGUR'ES 9 and 9a The evaluation of the signals produced by ultrasonic waves, for indicating deviations of the actual skin thick- 1 1 ness from a nominal skin thickness (FIGURES 2 and 3) and of the signals indicating deviations of the surface temperature of the strand from a nominal value is effected according to the block diagram of FIGURE 9. For the evaluation of these signals, the following adjusting functions are assumed.

(1) AT zero AK=zero+no control action AK=negativeincrease cooling AK=positiveincrease casting speed AT positive AK=zerono control action AK=negative+ increasing cooling .AK=positivereduce cooling AT negative AK=zero+ no control action AK=negative+reduce casting speed AK=positiveincrease casting speed.

On the left side of this table there is indicated the reference, and on the right side there is indicated the control action to follow this reference, whereby AT indicates deviations of the actual temperature of the strand surface from the nominal temperature and AK indicates deviations of the actual skin thickness from the nominal skin thickness. The signal forms of the different control blocks are visible in FIGURE 9a, where-by on the left side it results from the selected reference AT=negative and AK=positive the control action increase casting speed and on the right side from the selected reference AT=positive and AK=negative the control action increase cooling.

Due to the dendrites formed during the solidification and of other irregularities in the skin thickness substan tial fluctuations may appear in the signals indicating deviation from the nominal value. Such fluctuations can lead to a certain instability of the control, particularly if the produced signals of deviations are used directly to release the mentioned adjusting functions. For this reason in this example the pulses indicating deviations of the skin thickness from the nominal thickness are observed over a determinated time interval and only after a predeterminated excess of pulses departing in one direction from the nominal value a control order for adjusting is released.

The pulses H 1 for AK=positive and the pulses HQ for AK=negative act onto pulse counters with pre-sele'ction 150 and 151. The pulses H1 and H2 correspond to the pulses A12 and A13 in FIGURE 2b and B12 and B13 in FIGURE 3b. The counter 150 adds the pulses H1 and subtracts the pulses H2 while the counter 151 subtracts the pulses H1 and adds the pulses H2. As soon as in the counter 15!) and 151, respectively, the pulses H1 and H2, respectively, exceed a predeterminated value s, an outlet signal H3 and H4, respectively, is produced.

For the logistical connection of all control signals for the carrying out of the mentioned adjusting functions there are provided a NOR-gate 152, an OR-gate 153 and AND-gates 154, 155', 156, 157. An outlet signal H7 can be produced at gate 152 only if a signal H5 for AT=positive and a signal H6 for AT=negative miss simultaneously. The temperature signals H5 and H6 correspond to the signals E3 and E4 of FIGURE 6a or to the signals F8 and F9 of FIGURE 7a. At gate 103 an outlet signal H8 will appear only provided that one of the signals H6 or H7 is present. An outlet signal H9, H10, H11, H12 may appear at gates 154, 155, 156, 157, respectively, only provided that the signals H 4- and H7, H4 and H5, H6 and H8, H4 and H6, respectively, are present simultaneously.

The duration of the signals H9, H10, H11, H12 is different. For the stepwise actuation of the adjusting members it is desirable that these signals appear within a predeterminated time interval in order to obtain a constant adjusting action. This end the signals H9, H10, H11, H12 act onto a pulse former 158, 159, 1 60, 16-1, re-

spectively. The outlet signals H13, H14, H15, H16 of this pulse former appear with a selective duration k independently of the duration of the corresponding inlet signals. The period k must be of such length that With one evaluated control order the number of pulses on the counters and 151, respectively, is lowered beneath the pro-selected value.

A control order for the adjusting members has its effeet in the strand 14 only after a certain time. in order to make inactive pulses appearing in the meantime, there is provided a locking device. This device consists of elements 166, 16 7, 168 and 169. The NOR-gate 166 has an outlet signal H17 only when the signals H13, H14, H15 and H16 miss simultaneously. By means of the differentiating member 167 the signal H17 is transformed into a signal H18 which has its negative portion cut otf by a rectifier 16 8. The thus obtained outlet signal H19 acts onto the'time member 169 and switches-in the latter for the mentioned locking time h resulting in the formation of the control locking signal H20.

For the production of the definitive control order the signals H13, H14, H15, H16 and the control locking signal H20 act onto l-O-implication members 162, 163, 164, respectively. Each of the implication members 162, 16 3, 1 64 and 165 supplies its outlet signal H21, H22, H23 and H24, respectively, provided that the signal H13, H14, H15 and H16 is present and simultaneously the signal H20 disappears. The outlet signals H21, H22, H23 and H24 thus represent the control signals increase cooling, reduce cooling, increase casting speed and reduce casting speed, respectively. The outlet signals H21 and H22 act onto a motor .170 adjusting a cooling fluid valve 171 for the secondary cooling of the strand 14. The outlet signals H23 and H24 operate a motor 172 for adjusting the held excitation 17-3 of a Ward-Leonard-device associated to the rollers 16. The signals H2 1 and H22 produce in the motor a positive and negative rotation, respectively, for increasing and reducing respectively, the amount of cooling fluid at valve 171. The signals H2 3 and H24 result in the motor 172 into a positive and negative rotation, respectively, for increasing and reducing, respectively, the field strength of the Ward-Leonard motor so as to modify the casting speed. This modification of the casting speed due to its connection with further parts of the machine requires further adjustment such as the die oscillation, the speed of the straightening device, the speed of a rolling mill that may be connected after the continuous casting machine etc. All these adjustments are thus influenced by the signals signalling deviations of actual values from nominal values for the strand.

FIGURE 10 The evaluation of the signals produced by gamma-rays and indicating deviation of the actual skin thickness from the nominal skin thickness (FIGURE 4) and of the signals obtained by the electrical resistance method (FIG- URE 5) is effected in accordance with the block diagram of FIGURE 10. The corresponding signal forms are visible from FIGURE 10a. The adjusting functions are assumed to be the same as in FIGURE 9. Also the evaluation circuit is similar with the exception that the production of signals J3 and J4 from the signals 31 and 32 indicating deviations of the skin thickness is effected differently. Instead of the designation of the signals by H in FIGURE 9, in FIGURE 10 the signals are designated by J. In order to avoid instabilities in the control owing to the already mentioned irregularities in the thickness of the skin, it is possible instead of observing several pulses to maintain one signal for a certain time and to give an order for adjustment only after this delay time is passed. This determination of time interval is effected in the following example by a time element.

The signals 11 for AK=positive and J2 for AK=negative are transformed by differentiating members 1 80, i183,

into signal-s I25 and J27, respectively. The signals I25 and J27 switch in a time element 181 and 1-84, respectrvely, which switch olf after a certain time z. If the s gnals J25 or J27, disappear before the time z ends the time elements 181 and 1.84, respectively, are switched off immediately. The outlet signals of the time elements 181 and 184 are designated by J26 and J28, respectively. The signals J1 and J26 act onto a I-O-implication member 1 82 and the signals J2 and J28 act onto a I-O-implication member .185. At the implication members 182 and 185 outlet signals J6 and J4, respectively, may be produced only when on the one hand the signals 11 and J2, respectively are present and on the other hand the signals J26 and J28, respectively, are missing.

It is to be observed that a time interval k identical as to its function with the time interval k in FIGURE 9 is selected in such manner, that for each evaluated control order the signals J1 or J2, respectively, disappear.

The evaluation of the signals J3-J24 is effected in the same manner as that of the signals H-3-H24 in FIG- URE 9.

It is obvious that the invention is not limited to the few examples given herein'before. For example it is possible to measure the skin thickness for larger cross-sections in such manner that the measure of the skin thickness is determined for each side of the strand, whereby each of the individual measures or an average value may be used for influencing the adjusting tiunctions.

I claim:

1. The method of continuous casting which comprises the steps of continuously casting molten metal into a mold, cooling said mold to solidify the metal along the periphery to form a strand having a solidified skin enclosing a molten core, withdrawing said strand from said mold in continuous manner, cooling said strand to progressively solidify the molten metal in said core, measuring the thickness of the solidified skin of the strand enclosing the molten core and controlling the casting process in accordance with said measurement of the skin thickness to maintain the skin thickness enclosing the molten core with sufiicient material to withstand working operation according to the metal to be cast.

2. The method of continuous casting which comprises the steps of continuously casting molten metal into a mold, cooling said mold to solidify the metal along the periphery to form a strand having a solidified skin enclosing a molten core, withdrawing said strand from said mold in continuous manner, cooling said strand to pro gressively solidify the molten metal in said core, measuring the thickness of the solidified skin of the strand repeatedly, averaging said measurements thereby to overcome measurement distortion due to unevenness of and lack of homogeneity of said skin, and control-ling the casting process in accordance with said average of said measurements of skin thickness to maintain the skin thickness with sufficient material to withstand working operation according to the metal to be cast.

3. The method of continuous casting which comprises the steps of continuously casting molten metal into a mold, cooling said mold to solidify the metal along the periphery to [form a strand having a solidified skin enclosing .a molten core, withdrawing said strand from said mold in continuous manner, cooling said strand to progressively solidify the molten metal in said core, transmitting signals through said strand in a direction transverse to the axis of said strand, detecting signals reflected within said strand by the interfaces between solidified and molten metal, and .controlling the casting process in response to said detected reflected signals which serve as an indication of the location of such interfaces to maintain the skin thickness with sufiicient material to withstand working operation according to the metal to be cast.

4. The method of continuous casting (which comprises the steps of continuously casting molten metal into a mold, cooling said mold to solidify the metal along the periphery to form a strand having a solidified skin enclosing a molten core, withdrawing said strand from said mold in continuous manner, cooling said strand to progressively-solidify the molten metal in said core, measuring the thickness of the solidified skin of the strand at several positions displaced along the axis of said strand from the position of said first measurement and using the resulting measurements for controlling the casting process to maintain the skin thickness :with sufficient material to withstand working operation according to the metal to be cast.

5. A continuous casting plant comprising a continuous casting mold to receive molten metal and to solidify the periphery of said molten metal into a skin defining a strand and enclosing therewithin a molten core; means for pouring molten metal into said mold; means for withdrawing in continuous manner the strand cast by said mold; means for cooling said strand to progressively solidify the molten metal enclosed in the core of the strand; means for measuring the skin thickness of said strand after issuance of the strand from said mold; said measuring means comprising a source of ultrasonic signals, means for introducing said ultrasonic signals into said casting in a direction transverse to the longitudinal axis thereof, means for receiving said ultrasonic signals reflected from the interface between solidified and molten metal in said strand to determine the position of the interface between the solidified skin and the molten core; and means responsive to said measurement of skin thickness for controlling the casting process to maintain the skin thickness at that thickness corresponding to optimum casting speed and casting quality.

6. A continuous casting plant comprising a continuous casting mold to receive molten metal and to solidify the periphery of said molten metal into a skin defining a strand and enclosing therewith in a molten core; means for pouring molten metal into said mold; means for Withdrawing in continuous manner the strand cast by said mold; means for cooling said strand to progressively solidify the molten metal enclosed in the core of the strand; means for measuring the skin thickness of said strand after issuance of the strand from said mold; said measuring means comprising a source of ultrasonic signals, means for introducing said ultrasonic signals into said casting in a direction transverse to the longitudinal axis thereof, and means for receiving signals passing through said strand rfrom said source to determine the position of the interface between the solidified skin and the molten core in said strand; and means responsive to said measurement of skin thickness for controlling the casting process to maintain the skin thickness at that thickness corresponding to optimum casting speed and casting quality.

7. A continuous casting plant comprising a continuous casting mold to receive molten metal and to solidify the periphery of said molten metal into a, skin defining a strand and enclosing therewithin a molten core; means for pouring molten metal into said mold; means for withdrawing in con-tinuous manner the strand cast by said mold, means for cooling said strand to progressively solidify the molten metal enclosed in the core of the strand; means .for measuring the skin thickness of said strand after issuance or the strand from said mold; said measuring means comprising means for the generation of a signal, fluid coupling means coupling said signal generator to said strand to introduce said signal into said casting in a direction transverse to the longitudinal axis thereof, and means for receiving said signal to determine the position of the interface between the solidified skin and the molten core in said strand; and means responsive to said measurement 0t skin thickness for controlling the casting process to maintain the skin thickness at that thickness corresponding to optimum casting speed and casting quality. 

1. THE METHOD OF CONTINUOUS CASTING WHICH COMPRISES THE STEPS OF CONTINUOUSLY CASTING MOLTEN METAL INTO A MOLD, COOLING SAID MOLD TO SOLIDIFIED SKIN ENCLOSING A MOLTO FORM A STRAND HAVING A SOLIDIFIED SKIN ENCLOSING A MOLTEN CORE, WITHDRWAING SAID STRAND FROM SAID MOLD IN COMTINUOUS MANNER, COOLING SAID STRAND TO PROGRESSIVELY SOLIDIFY THE MOLTEN METAL IN SAID CORE, MEASURING THE THICKNESS OF THE SOLIDIFEIED SKIN OF THE STRAND ENCLOSING THE MOLTEN CORE AND CONTROLLING THE CASTING PROCESS IN ACCORDANCE WITH SAID MEASUREMENT OF THE SKIN THICKNESS TO MAINTAIN THE SKIN THICKNESS ENCLOSING THE MOLTEN CORE WITH SUFFICIENT MATERIAL TO WITHSTAND WORKING OPERATION ACCORDING TO THE METAL TO BE CAST. 